Neuroinflammation following TBI exacerbates neuronal and glial death and dysfunction. The monocyte- derived macrophage response is harmful in TBI. However, macrophages are a broadly heterogeneous population and polarized subsets can be critical for wound repair. Our goal is to identify harmful and beneficial components of the innate immune response to TBI and to define when they arise, to identify optimal targets and timing for therapeutic intervention. The ultimate goal is to block harmful responses and enhance reparative ones to improve long-term functional outcomes of TBI. Our recently published study showed that Ccr2-/- mice had reduced monocyte-derived brain macrophages and improved outcomes post-TBI. We hypothesize that pharmacological blockade of human CCR2 will improve TBI. Our new data shows that a CCR2 antagonist, CCX872 (in Phase II clinical trials for nephropathy), given 5 min after TBI in mice expressing human CCR2 blocked 832% of the macrophage infiltrate. Aim 1. Determine the therapeutic benefit of the human CCR2 antagonist, CCX872, given after TBI, to mice expressing human CCR2 to inhibit macrophage infiltration into the brain, and to improve pathologic and behavioral outcomes. 1a. Determine the effective dose range and time window of hCCR2 antagonist, CCX872, administered after TBI to block macrophage infiltration to the brain. 1b. Determine the effective dose range and time window of hCCR2 antagonist, CCX872, administered after TBI to improve behavioral functions and histopathology. Our previous published work and our new preliminary single-cell RNA sequencing studies indicate that TBI macrophages are diverse and are not limited to the M1/M2 paradigm. We hypothesize that multiple monocyte- derived macrophages and microglia subsets develop after TBI. Aim 2. Define the diversity of monocyte-derived macrophages and resident microglia in the brain post- TBI by the use of single-cell, whole-transcriptome analysis. 2a. Define the monocyte-derived brain macrophage subsets post-TBI by performing single-cell RNA seq. 2b. Define microglia subsets post-TBI by performing single-cell RNA seq. In the absence of CCR2, a macrophages subset still infiltrates the injured brain after TBI, and is associated with improved outcomes. We hypothesize that CCR2-independent macrophages are protective in TBI. Aim 3. Determine the expression profile, functional role, and requirement for CCR2-independent macrophages in post-TBI outcomes. 3a. Define CCR2-independent macrophage subsets in the brain after TBI by single-cell RNA seq. 3b. Define the functional nature of the CCR2-independent macrophage response to TBI by examining phagocytosis, cytokine production, and T cell suppressor function by these cells. 3c. Determine if CCR2-independent macrophages are beneficial or harmful by depleting them with clodronate liposomes in Ccr2-/- mice after TBI and determining the effects on TBI outcomes. This work will provide the preclinical basis and design for targeting CCR2 in TBI therapy. Furthermore, the work will provide an unprecedented view of macrophage biology in the specific context of TBI. These studies will lead to informed approaches for altering neuroinflammation to improve outcomes for veterans.